Direct drive electromechanical linear actuators

ABSTRACT

Direct drive electromechanical rotary-to-linear actuators include one or more electric motors mounted in a housing. Each motor includes a stator and a rotor. The motor drives a planetary drive mechanism that includes an elongated central shaft having one or more helical threads on an external surface coupled to the rotor for conjoint rotation. A planetary nut having helical threads on an internal surface is disposed concentric to the shaft, and a plurality of planetary rollers are disposed concentrically between the shaft and the planetary nut. Each of the rollers has a helical thread on an external surface that is complementary to and in engagement with a thread of the shaft and a thread of the nut. Rotation of the rotor is converted with mechanical advantage into linear movement of the planetary nut. The actuators provide backlash-free operation, higher stiffnesses, slew rates and frequency responses, and better overall efficiency.

TECHNICAL FIELD

This invention relates to motion control or actuation devices ingeneral, and in particular, to direct drive electromechanicalrotary-to-linear motion actuators.

BACKGROUND

Electromechanical actuators are rapidly displacing hydraulic devices ina wide range of industries, including aviation. Improvements in solidstate switching devices, their digital control and in the performance ofmagnetic materials have all contributed to an increased interest inelectric actuators. Electromechanical linear actuators are particularlywell suited to flight control applications, as well as a multitude ofindustrial uses, particularly in production automation. Automotive andother vehicle applications also abound, as may be found, for example, inthe variable transmissions and caliper or disk brake actuators describedin, e.g., U.S. Pat. Nos. 6,837,818 and 6,626,778 to Kapaan et al., and6,367,597 and 6,318,512 to De Vries et al.

Flight applications, such as actuation of control surfaces, helicopterrotor blades, lift enhancement devices, landing gear deployment andbraking, door opening, and the like, are all best handled with linearactuators. Rotary-to-linear conversions, correctly engineered, canprovide backlash-free operation, high stiffness, high slew rates, goodoverall efficiency and high frequency response, which properties are allneeded in combination for an ideal flight control actuator.

Concomitant with their widespread adoption, certain shortcomings invarious mechanical aspects of the devices have also become apparent, andtherefore merit attention. These problems include backlash, wear,complexity and cost and life limitations.

Accordingly, there is a long-felt but as yet unsatisfied need in anumber of fields for linear actuators that overcome the backlash, rapidwear, complexity, high cost and limited life problems incident to priorart actuators, and that provide backlash-free operation, higherstiffnesses, slew rates and frequency responses, and better overallefficiency.

BRIEF SUMMARY

In accordance with the various exemplary embodiments of the inventiondescribed herein, direct-drive electromechanical rotary-to-linearactuators are provided that address and overcome many of the above andother shortcomings of prior art actuators.

In one exemplary embodiment thereof, a direct drive electromechanicalrotary-to-linear actuator includes an elongated housing and an electricmotor, including a stator fixed in the housing and a rotor supported forrotation relative to the stator, coupled to a planetary drive unit. Thedrive unit comprises an elongated central shaft coupled to the rotor forconjoint rotation therewith and has a plurality of helical threads on anexternal surface thereof. A planetary nut having a plurality of helicalthreads on an internal surface thereof is disposed concentric to theshaft, and a plurality of planetary rollers, each having a helicalthread on an external surface thereof that is complementary to and inengagement with both a thread of the shaft and a thread of the nut, isdisposed concentrically between the shaft and the planetary nut.

In one advantageous variation thereof, the central shaft of the actuatoris made integral to the rotor of the motor and constructed of amagnetically permeable material, e.g., an alloy steel. The stator cancomprise high-cobalt-content laminations and the rotor can compriseneodymium-iron magnets.

In another variation, the central shaft can include a hollow center,with a linear variable displacement transducer (“LVDT”) disposed thereinfor measuring the absolute position of an output end of the actuatorrelative to a fixed end thereof.

In another variation, a plurality of axial grooves can be provided inthe housing adjacent to the planetary nut, and a plurality of axialsplines can be provided on the nut, each of which is slidably disposedin a respective one of the grooves to prevent rotation of the nutrelative to the housing and thereby react any torque on the nut toadjacent machine structure.

In another variation, the elongated rotor is of common material to theplanetary nut housing which is coupled to the annular shaft within theannulus thereof to conserve actuator length and provide a double-endedactuator if desired. An absolute-angular-position encoder can berotatably coupled to the central shaft for detecting the absolute linearposition of the shaft.

In another exemplary embodiment, the actuator can include a secondelectric motor, e.g., a backup motor, including a second stator fixed inthe housing and an elongated annular second rotor supported for rotationrelative to the second stator. The associated planetary drive unit ofthis embodiment comprises a planetary nut that is coupled to the annularsecond rotor within the annulus thereof for both conjoint rotation andrelative axial sliding therein. As above, a plurality of threadedplanetary rollers is disposed concentrically between the central shaftand the planetary nut, and means are provided for selectably locking therespective rotors of the two motors against rotation, thereby providinga “fail operational” mode of the actuator in the case of amalfunctioning main motor.

In one possible alternative implementation of this embodiment, at leastone of the two motors, e.g., the backup motor, comprises a “pancakemotor,” and the malfunctioning main motor locking means can comprise adouble-armature, double-disc, solenoid clutch brake operable toselectably lock/unlock respective ones of the two rotors.

In another alternative implementation, e.g., in a high power actuatorapplication, the rotor locking means can comprise a clutch plate mountedon the rotor of the main motor for conjoint rotation and positioned suchthat it rotates between the jaws of a caliper brake. A plurality ofrocking levers having rollers disposed at respective first ends thereofare arranged to move between a first orientation, in which the rollersengaged in respective slots in an end of the rotor of the backup motor,and a second orientation in which the rollers are disengaged from theslots. A plurality of first springs biases the rocking levers into thefirst orientation, such that the backup motor is locked against rotationduring normal operation. A second spring, e.g., a belleville spring,biases the jaws of the caliper brake together and against the clutchplate on the main rotor, to thereby clamp the rotor against any rotationduring a malfunction. A spool is moveable between a first positioncompressing the second spring and thereby relieving the bias of thesecond spring on the caliper brake jaws during normal operation of themain motor, and a second position locking the main motor and urging therocking levers against the bias of the first springs and into the secondorientation thereof, thereby unlocking the backup motor for rotation.

Means are provided for releasably holding the spool in the firstposition during normal operation of the actuator. In a simple yetreliable implementation, these means can comprise a plurality of ballbearings disposed in apertures in the spool and held captive in anadjacent circumferential groove by an arm of a solenoid. The samesolenoid can be used to selectably release the spool from the firstposition for movement to the second position, e.g., in response to amalfunction of the main motor, whereupon the actuator rapidly switchesto a “fail operational” mode in which the backup motor takes overoperation of the actuator from the malfunctioning main motor.

In yet another exemplary embodiment that is advantageously adapted tominiaturized actuator applications, the actuator can comprise anelectric motor that includes a stator supported for axial movement in astator housing, and a rotor supported in the housing for conjoint axialmovement with, and rotation relative to, the stator. The planetary driveunit of this embodiment comprises an elongated, narrow, unthreadedcylindrical spindle having a long axis and coupled to the rotor forconjoint rotation therewith. In the place of a planetary nut, a drumhaving a thin, cylindrical sidewall with a plurality of helicalcorrugations therein is disposed concentric to the spindle. In the placeof elongated, planetary rollers, a plurality of disc-like rollers isdisposed in a radially symmetrical array about the spindle. Each of therollers is mounted on the stator housing for rotation about an axis thatis skewed at an angle equal to the pitch angle of the drum corrugations,and each has a circumferential surface with convolutions that correspondto the corrugations in the drum. The circumferential surface of each ofthe rollers is disposed in frictional engagement with both the spindle arespective one of the corrugations of the drum.

In an alternative embodiment, the rollers can be located in adjacentplanes perpendicular to the spindle, each plane containing a radiallysymmetrical array of two or more rollers. In another advantageousvariation, the wall of the drum can be strained from a cylindrical shapeto a trochoidal shape incorporating a plurality of longitudinal zoneshaving a smaller radius alternating with a plurality of longitudinalzones having a larger radius. The resulting beam-bending thereby imposedin the wall of the drum, coupled with the stiffening effect of thecorrugations therein, provides an inward-directed restoring force thatengages the rollers in diametral compression between the walls of thedrum and the motor spindle.

A better understanding of the above and many other features andadvantages of the present invention may be obtained from a considerationof the detailed description of the exemplary embodiments thereof below,particularly if such consideration is made in conjunction with theappended drawings, wherein like reference numerals are used to identifylike elements illustrated in one or more of the figures therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side elevation view of a first exemplaryembodiment of a direct drive electromechanical linear actuator inaccordance with the present invention;

FIG. 2 is a cross-sectional side elevation view of a second exemplaryembodiment of a direct drive electromechanical linear actuator inaccordance with the present invention;

FIG. 3 is a cross-sectional side elevation view of a third exemplaryembodiment of a direct drive electromechanical linear actuator inaccordance with the present invention;

FIG. 4 is a cross-sectional side elevation view of a fourth exemplaryembodiment of a direct drive electromechanical linear actuator inaccordance with the present invention, in which a clutch brake mechanismthereof is shown in the encircled area 5-5;

FIG. 5 is an enlarged detail view of an alternative embodiment of aclutch brake mechanism to that shown in the encircled area 5-5 of FIG.4;

FIG. 6 is a cross-sectional side elevation view of a fifth exemplaryembodiment of a direct drive electromechanical linear actuator inaccordance with the present invention;

FIG. 7 is a cross-sectional view of the actuator of FIG. 6, as takenalong the lines 7-7 therein;

FIG. 8 is an enlarged cross-sectional detail view of a roller of theactuator of FIG. 6, as taken along the lines 8-8 therein; and,

FIG. 9 is another enlarged cross-sectional detail view of a roller ofthe actuator of FIG. 6, as taken along the lines 9-9 therein.

DETAILED DESCRIPTION

This invention discloses various exemplary embodiments of novel,direct-drive, electromechanical, rotary-to-linear actuators. Asvariously illustrated in the figures, the exemplary mechanism thatconverts rotation to translation in the various embodiments is ahelically threaded, planetary-nut-and-shaft arrangement of a type suchas is described in U.S. Pat. No. 3,884,090 to L. I. Dock. Unlike aconventional nut/thread combination, such an arrangement comprises threeelements, viz., 1) a central shaft, or lead screw, containing one ormore external helical threads; 2) a grouped set of rollers or “planets”incorporating a corresponding external helical thread; and, 3) a “nut,”or housing containing a plurality of corresponding internal helicalthreads circumscribing the former two elements. These elements can berespectively analogized to those of a conventional concentric planetarygear arrangement of a type that includes a “sun gear,” a set of“planetary gears,” and a “ring gear” circumscribing the two formerelements. However, as discussed below, such planetary gear trains havecertain drawbacks that are overcome by the present invention. Further,as will be appreciated by those of skill in the art, other types of nutarrangements, such as a ball-screw-and-nut, acme-thread-and-nut,recirculating-ball-nut, or plain thread and nut can also function tosome effect, and may be substituted for the aboveplanetary-nut-and-shaft arrangement in some of the embodiments describedherein.

As shown herein, when driven by an electric motor, or motors, severaladvantageous embodiments incorporating such planetary-nut-and-shaftarrangements are possible, and provide useful mechanical or reliabilityimproving outcomes not previously found in prior art actuators. Thisinvention is thus directed to non-geared arrangements of components,sometimes referred to as “direct drive” actuators. The use of directdrive avoids the backlash, complexity, efficiency and reliability issuesassociated with prior art actuators that incorporate gear trains. Thus,while some gear tooth forms may appear in some of the embodimentsdescribed and illustrated herein, it should be understood these are usedfor alignment purposes only, and not for power transmission purposes.For example, the rollers in a planetary nut may have small gearsdisposed at each end that mesh with corresponding internal sets of teethat corresponding ends of the nut. It should be understood that thesefunction only to maintain the rollers parallel to the shaft and nutduring operation of the device, and are not used to transmit eithertorque or thrust.

FIRST EXEMPLARY EMBODIMENT

Turning now to the figures, a first exemplary embodiment of a directdrive electromechanical linear actuator 100 in accordance with thepresent invention, comprising a single motor, single-ended device, isillustrated in the cross-sectional side elevation view of FIG. 1. Theactuator 100 comprises a brushless electric motor 102 having a stator104 mounted in an elongated housing 106. A rotor 108 incorporatingpermanent magnets 110 and an elongated, externally threaded centralshaft, or lead screw 112, is rotatably supported in the housing by a setof bearings 114. The rotor 108 supports a motor encoder 116 of a knowntype, which functions as a substitute for a motor commutator, forconjoint rotation with the rotor. A tubular output extension 118 havingan output end 120, and containing an internally threaded planetaryroller nut 122 located at the opposite end thereof, is disposedcoaxially within and guided by the housing for both rotation about andtranslation along the long axis of the actuator. A set of externallythreaded planetary rollers 124, each having one or more threadssimultaneously engaging corresponding threads in both the lead screw 112and the planetary roller nut 122, is disposed coaxially between thelatter two features for both rotation about and translation along thelong axis of the device.

In use, the housing 106 of the actuator 100 is attached to adjacentmachine structure (not illustrated), and the output end 120 is attachedto the component to be actuated (not illustrated). Optionally, a linearvariable displacement transducer (“LVDT”) 125, comprising a magneticallypermeable rod 126 disposed concentrically within a pair ofelectromagnetic coils 127 contained in the central shaft 112, is mountedfor relative axial movement within the coils to sense the absoluteposition of the output end of the actuator relative to the other, fixedend 121 thereof. In this, the simplest of actuator configurations, thetorque developed by the motor 102 and transmitted to the output end 120via the central shaft and nut 122 is reacted by adjacent structure (notillustrated), and optionally, at the output end by the actuated device,such that the nut is prevented from rotating, and hence, limited to onlytranslational motion along the long axis of the device.

A close examination of the actuator 100 of FIG. 1 reveals that thestructure of the rotor 108 can be implemented in two alternativeconfigurations. In the first of these, the lead screw 112 portion of therotor can comprise a separate, externally threaded annular tube 128 thatis supported at one end, e.g., by a shrink fit technique, concentricallywithin, and thereby coupled directly to, a separate annular rotorportion 130, as illustrated in the half-portion of the figure above thecenterline of the actuator of FIG. 1. Alternatively, the lead screw androtor portions can be manufactured integrally from a single piece ofmagnetically permeable material, as illustrated in the half-portion ofthe figure below the actuator centerline. The high-torque motor 102 ispreferably constructed from high-cobalt-content laminations used in thestator 104 and neodymium-iron magnets 110 attached to the rotor 108.While the resulting motor is slightly larger than that which might beexpected for an actuator of a comparable output force whose motor drivesthrough a reduction gearset, it should be noted that no gears orbearings are required in the actuator, other than those used to supportthe rotor/shaft components for rotation, as described above.Consequently, the foregoing combination of features, together with theelimination of any gear trains, results in an overall savings inactuator cost and weight, enhanced actuator life and reliability, acomplete elimination of backlash, a gain in slew rate speed, andimproved actuator stiffness and frequency response.

SECOND EXEMPLARY EMBODIMENT

A second exemplary embodiment of a direct drive electromechanical linearactuator 200 in accordance with the present invention, comprising asingle motor, single-ended device with internal torque reaction, isillustrated in the cross-sectional side elevation view of FIG. 2. As maybe seen by a comparison of the respective first and second embodimentsof FIGS. 1 and 2, the second embodiment of actuator 200 incorporatesseveral of the features of the first embodiment of actuator 100, butwith the addition of a nut-and-output-end anti-rotation feature 232 and234, as illustrated in FIG. 2. In particular, in the second embodiment,low-friction (e.g., Teflon) splines, or sliders 232, are disposed on theplanetary nut 222 to slide with a slight interference fit incomplementary longitudinal grooves, or tracks 234, disposed in thehousing 206. The motor 202, and hence, the torque on the nut 222, isthereby reacted directly back to the housing, in a manner that preventsany rotational backlash between the nut and housing.

As a result of this arrangement, the position-to command fidelity of theactuator 200 is not affected by any free movement that might be presentin another type of arrangement, e.g., a gear train arrangement. Thisembodiment of the actuator can thus be mounted on ball joints (e.g., asa pin-jointed link) at either or both ends 220 and 221 thereof, and canbe used, e.g., between machine parts that move along different axes,because all torque reactions occur internally of the actuator. Optionalelastomeric anti-jamming travel stops 236 and a sliding seal 238disposed around the tubular output extension 218 can also be provided,as shown in FIG. 2, to softly limit the end positions of the outputextension and exclude dirt and moisture from the interior of the device.

The difference in the material bulk moduli between the low-frictionsliders 232 and the complementary housing tracks 234 within which theyslide ensures that the relative movement of the fixed and moving partsof the actuator 200 is accomplished with low frictional losses and acomplete avoidance of any clearance over a long life span. This enablesbacklash-free operation of the actuator and the achievement ofhigh-frequency motion.

THIRD EXEMPLARY EMBODIMENT

A third exemplary embodiment of a direct drive electromechanical linearactuator 300 in accordance with the present invention, comprising asingle motor, and either a single- or double-ended device with enhancedtravel range, is illustrated in the cross-sectional side elevation viewof FIG. 3. As in the first and second embodiments above, a brushlesselectric motor 302 includes a stator portion 304 fixedly mounted in ahousing 306. A rotor portion 308 is also rotatably supported in thehousing by a set of bearings 314, and the rotor portion may also supporta motor encoder 316 for conjoint rotation, as in the embodiments above.

As illustrated in FIG. 3, the rotor portion 308 of the motor 302 of thethird embodiment is made as an elongated annular shaft on whichneodymium-iron magnets 310 are mounted, as in the above embodiments, andin which the nut 322 of the planetary roller screw is concentricallymounted. The non-rotating central shaft, or lead screw 312, has a clevisarrangement 340 disposed at one or both ends thereof to attach thecomponent(s) to be actuated. The lead screw itself can also be madehollow internally, as shown, for weight saving. Anabsolute-angle-position encoder 342 for measuring the absolute axialposition of the central shaft is rotatably coupled to the shaft with ahelical gear (not illustrated) engaging the threads of the lead screw.

As may be seen by reference to FIGS. 1, 2 and 3, the total travellength, as well as the structural length of the actuator 300 issubstantially increased, relative to those of the first and secondembodiments of actuator 100 and 200 described above. In particular, ifactuator “dead length” is defined as the sum of all the length elementsof the actuator that do not contribute to travel, then actuators with“closed” centers will have a maximum length that is equal to their deadlength plus their travel length, whereas, actuators with “open” centerswill have a maximum length that is equal to their dead length plus twicetheir travel length. This characteristic of the first and secondembodiments described above, which are closed center devices, makes thestructural length and weight of long-travel actuators an issue ofincreasing concern as the length of travel of the device increases.However, the third embodiment of actuator 300, having open centers, hasa constant and compact dead length, making the device more suitable forlong-travel applications. In this embodiment, motor torque must betransferred from the housing 306 to adjacent machine structures (notillustrated), and hence, to the actuated devices (not illustrated)mounted to the opposite ends of the central shaft 312 to complete theload path of the actuator.

It may be further appreciated in connection with the third exemplaryembodiment of actuator 300 that, by combining the nut 322 feature of aplanetary roller screw arrangement directly with a hollow rotor portion308 of an electric motor, and further, by combining the nut as anintegral part of the magnetic material of the rotor, as illustrated inthe alternative cross section of the rotor half-portion below thecenterline of the actuator of FIG. 3, such combination enables thepassage of a lead screw 312 having a relatively long length, which islimited only by the stiffness, stability and support requirements of thedevice. The actuator 300 is therefore particularly well suited tolong-travel applications.

FOURTH EXEMPLARY EMBODIMENT

A fourth exemplary embodiment of a direct drive electromechanical linearactuator 400 in accordance with the present invention, comprising a dualmotor, single output, internal-torque-reacting actuator with asubstantial degree of redundancy, is illustrated in the cross-sectionalside elevation view of FIG. 4. Of importance, the fourth embodiment ofactuator 400 comprises a pair of brushless electric motors, viz., a mainmotor 402A and a backup motor 402B. As illustrated in FIG. 4, therespective stators 404A, 404B of the two motors are fixedly mounted inthe actuator housing 406. The housing also mounts the bearings sets 414Aand 414B for the respective motor rotors 408A and 408B, respective motorencoders 416A and 416B, and in the particular alternative embodimentillustrated in FIG. 4, the housing also mounts the windings (notillustrated) for a double-armature, double disc solenoid clutch brake442A, as well as a set of thrust bearings 444 for the central shaft, orlead screw 412, of the actuator.

As may be seen in FIG. 4, the lead screw 412 is common to and anextension of the rotor 408A of the main motor 402A. A rotating supportelement 446 with internal axial grooves 432 supports the planetaryroller nut 422 and is formed common to and as an extension of the rotor408B of the backup motor 402B. The roller nut itself includeslow-friction sliders 432 that slide within the grooves 434 of therotating support element for reacting torque internally within theactuator 400, as described above in connection with the secondembodiment of actuator 200, and is disposed in the tubular actuatoroutput extension 418, which terminates in a clevis 440 mounted on thrustbearings 446 at the output end 420 of the actuator. The actuator isenvironmentally sealed with a sliding resilient seal 438 disposed aroundthe tubular output extension and, with ball joints (not illustrated)fitted at each end thereof, functions as a pin-jointed link withoutreacting any torque to its mounting structure. Clutch plates 448A and448B of the solenoid clutch brake 442A, comprising a high-frictionmaterial, e.g., carbon-graphite and/or asbestos, are attachedrespectively to the rotors of the main and backup motors.

During normal operation of the fourth embodiment of actuator 400, themain motor 402A has the lower inertia of the two motors and is thereforepreferred for high-frequency operation. By contrast, the backup motor402B is preferably configured in a “pancake” form, i.e., one having arelatively shorter length and a relatively larger diameter, so that theplanetary nut 422 and sliders 432 can operate within its insidediameter, thereby conserving actuator length. In normal operating mode,the clutch brake 442B of the backup motor 402B is spring loaded toengage and lock the rotor. The slider grooves 434 of the nutanti-rotation element 446 are thus held static, and nut torque isnormally reacted through the engaged backup motor clutch. The clutchbrake for the main motor is spring loaded to be normally disengaged, andis thus normally out of contact with the main rotor.

The main and backup motors 402A and 402B of the actuator 400 areprovided with independent power, motor control and motor-encodingcircuits. A malfunction of the main motor or its control will show as anincorrect or nonexistent response to a position command. This detectionis extremely rapid. For example, the main motor could be shorted,stationary, moving slowly, dithering, or accelerating at full power toan un-commanded position. In all such malfunction scenarios, it isnecessary that the malfunctioning motor be frictionally arrested asquickly as possible. To effect this, the solenoid clutch brake 442A iselectrically activated and latched to a “backup” state, namely,malfunctioning motor locked, and backup motor freed. Accordingly, theclutch brake is sized to arrest the full torque of a powered motor. Thesolenoid windings of the brake are likewise sized to permit beingenergized constantly, and hence, to produce the coercive force neededfor engagement of the appropriate friction plate 448A or 448B withoutoverheating.

In this “fail operational” mode, the actuator 400 now operates with arotating nut 422 riding on a static central shaft 412, and functionsnormally, although with reduced frequency response because of theinertia of the larger-diameter components in the “backup” path.Operation of the single LVDT 425 is unaffected by the drive path change.Notably, both the roller screw mechanism and the LVDT are considered tobe of such high reliability as to not warrant redundancy. If, afterinvestigation, the fault is found to be in the prime path motorcontroller, this embodiment of actuator will revert without any externalattention (i.e., by absence of a solenoid command) to normal operationvia the main motor 402A.

As may be seen from the foregoing, the fourth exemplary embodiment ofthe actuator 400 combines many of the benefits of the first, second andthird embodiments described above, namely, freedom from backlash, highslew rate, high system stiffness and long life, with a substantialdegree of redundancy by incorporating the space-saving benefits of abackup motor 402B, shaped as a “pancake,” with internal space usedadvantageously for the above planetary nut-and-slider arrangement. Alsoincorporated is a unique packaging arrangement in which the respectivelamination cores of the solenoid windings (not illustrated) of therespective rotor clutch brakes are disposed back-to-back in a rigid,monolithic structure that is attached to the housing, and thus serves asa robust load path for transferring the substantial axial loads imposedby the main motor 402A and the thrust and backup motor bearings 444 and414B to the housing, thereby enabling a substantial reduction in size tobe achieved, with the result that the redundant actuator is only about20% longer than the single-motor actuator 200 of the second embodimentdescribed above.

An alternative implementation of the double-armature, double-discsolenoid clutch brake mechanism 442B of the fourth embodiment ofactuator 400 is illustrated in the enlarged detail view of FIG. 5. Inthis alternative implementation, the redundancy features of the actuator400 are modified by redefining the concept, method and manner ofeffectuation of the drive conversion process in going from the Main tothe Fail Operation modes, to enhance the suitability of the actuatordesign to very large devices of 20 kW power and larger, as well as foruse in manned-flight operations.

FIG. 5 (viewed in conjunction with FIG. 4), illustrates the layout ofthe alternative clutch brake 442B components disposed between the mainand backup motors 402A and 402B, which, as discussed above, are ofconventional and pancake configurations, respectively. As above, thehousing 406 contains the stators 404A and 404B of both motors, splines450 for mounting static clutch plates 452 associated with the mainmotor, and mountings for a tangentially positioned solenoid 454, and thethrust bearing sets 414A and 414B of both the main and backup rotors,respectively.

The main motor rotor 408A is radially extended and splined to accept arotating clutch plate 456 of a high-friction material disposed betweenthe jaws of a caliper clutch brake 457. In contrast, the backup motorrotor 408B is axially extended to form a drum 458 having axial slots atan end thereof. The rotating clutch plate of the main rotor isalternately locked against, or unlocked for, rotation about the centralaxis by means of the compressive force of a belleville spring 460 actingagainst a moveable one of the jaws of the caliper clutch brake. Incontrast, the backup rotor is alternately locked against, or unlockedfor, rotation by the engagement or disengagement of rollers 462,disposed at the respective ends of a plurality of reciprocating rockinglevers 464, in the respective axial slots in the end of the backup rotordrum extension in the following manner.

As shown in FIG. 5, the rollers 462 are mounted at one end of therespective rocking levers 464. A plurality of first springs 465resiliently bias respective ones of the other ends of the rocking leverssuch that the rollers at the first ends respectively engage the axialslots in the end of the backup rotor 408B drum extension 458, therebylocking it against rotation. The space between the belleville spring 460and the inner ends of the rocking levers is occupied by a flanged,annular shuttle, or spool 468, capable of axial sliding on a concentriccylindrical surface that contains an external annular locking groove470. An arm 472 of the solenoid 454, which is arranged to movetangentially and rotate in-plane, holds individual ball bearings 474captive in the annular locking groove, thereby preventing axial movementof the spool. In this holding position, the spool holds the bellevillespring in a compressed state, thereby relieving any pressure by thespring on the jaws of the caliper brake 457 and thus freeing the clutchplate 456 of the main rotor 408A for rotation, and at the same time,through its contact with the inner ends of the rocking levers, holdingthe rollers at the outer ends of the rocking levers in lockingengagement with the slots in the end of the backup rotor drum extension,as illustrated in the half-portion of the actuator above the centerlineof FIG. 5. When disposed in this locked position, all components aremechanically inert, and the state of the actuator 400 cannot changeexcept by outside intervention, namely, by the transmission of an unlockcommand to the solenoid.

Those of skill in the art will appreciate that, as the size, power andhence, torque rating of an actuator increases, the ability of adirect-acting solenoid clutch combined with a friction element (as inthe alternative clutch brake embodiment of FIG. 4) to react torqueeffectively becomes increasingly limited. In the exemplary 20 kWactuator 400 considered here, torques are on the order of 250 ft-lb_(f).Accordingly, the state change that occurs in going from the main to thebackup mode upon fault detection is desirably irreversible, except bysubsequent external intervention for re-setting of the actuator. Theadvantages that result from the set-state being triggered to changeirreversibly are particularly applicable to high-power actuators.

By definition of the nut-reaction torque path, the rotor 408B of thebackup motor 408B is a statically reacting torque. An effective,reliable method of releasing this torque is by means of the rollers 462positioned with their respective rolling axes disposed normal to theplane of the applied force. As described above, the main motor 402Afault mode can be one of being stalled, moving slowly, dithering, orhaving a full torque speed excursion. In all such cases, the main rotor402A is best arrested by using a clutch plate 456 having a highcoefficient of friction (e.g., carbon-on-carbon), clamped between thejaws of the caliper brake 457 by a high-force spring. One advantageouscharacteristic of the belleville spring 460 is that its release forceincreases non-linearly with travel. This means that the force requiredto hold the main rotor in the unlocked state (effected through themechanism of the spool 468, annular groove 470 and ball bearings 474) isrelatively low, while the force available to lock the main rotor and torelease the rollers 462, thereby unlocking the backup rotor, isrelatively much higher. The large force margin (6:1) guarantees backuprotor rotation within milliseconds of the state-change command to thesolenoid 454. The unlocked, or fail operational, configuration of thealternative clutch brake 442B, i.e., after the malfunctioning main rotorhas been arrested and locked against rotation, and the backup rotorunlocked for rotation, is as illustrated in the half-portion of theactuator below the centerline of FIG. 5.

Thus, making the state change from prime path motor drive to backup pathmotor drive of the alternative embodiment of the actuator 400irreversible (except by subsequent intervention) enables not only theability to use different clutch types that are best suited to theparticular conditions of the respective rotor release or engagement, butalso the ability to release and engage two clutches simultaneously usingonly one primary belleville spring 460 and a low-energy-signal solenoid454. This alternative arrangement therefore offers extremely highmechanical reliability by virtue of its inert, prime-path-locked state.

FIFTH EXEMPLARY EMBODIMENT

A fifth exemplary embodiment of a direct drive electromechanical linearactuator 500 in accordance with the present invention, comprising asingle-motor linear actuator with helical-roller stator and otherfeatures suitable for miniaturization of the actuator, is illustrated inthe cross-sectional side elevation view of FIG. 6. In place of theelongated planetary rollers of the previous embodiments, the exemplaryactuator 500 incorporates a plurality, e.g., four, disc-like rollers 524mounted for independent rotation on a stator casing 506 and disposed ina radially symmetrical “star” arrangement in which the common diametersof the respective rollers are such that they overlap and require twoplanes of location to accommodate two sets each of two diametricallyopposite rollers, as illustrated in the cross-sectional view of FIG. 7.

The rollers 524 are mounted in ball-bearings 566 and their respectiveaxes of rotation are mutually inclined in the same direction at a commonhelix angle φ, as illustrated in the enlarged detail view of FIG. 8. Anelectric motor 502 is slidably supported in the stator casing 506 bysplines 568 that move in axial keyways 570 such that the motor iscapable of limited axial movement in the casing, but incapable ofrotation therein. To accommodate this small axial movement of the motor,power and control signals may be conducted to the motor via extensibleand retractable “service loop” wires 572. The spindle, or central shaft512, of the motor comprises a precision-made, hardened cylinder having arelatively small diameter and a circumference that frictionally engagesthe circumferential surface of each of the rollers, as illustrated inFIG. 7.

Convolutions 523 in the circumferential surface of each of the starrollers 525 engage in corresponding helical corrugations 574 in thethin-walled, cylindrical side wall of a hardened drum 522, wherein thehelix angle φ matches the angle of inclination φ of the roller axes, asillustrated in the detail view of FIG. 9. The hardened rollers havetheir rims configured with two convolutions of a pitch given by πD tanφ, where φ is the helix angle and D is the drum diameter. As an example,in the case of a 10 mm diameter drum and a 1.5 degree helix angle φ, theconvolution pitch is 0.82 mm. The crests of the respective convolutionsare truncated with a true cylindrical portion. During assembly, the drumis preferably strained slightly from a true cylindrical configuration toa more trochoidal shape, yielding four longitudinal zones of a smallerradius, interspersed with four longitudinal zones of a larger radius.The resulting beam-bending thereby imposed on the shell of the drum,coupled with the stiffening effect of the corrugations therein, providesan inward-directed restoring force that engages the rollers in diametralcompression between the shell and the motor spindle 512.

The star roller mountings of the stator 508 are axially stiff, butradially are just sufficiently compliant to allow full contact betweenthe rollers 524 and the motor spindle 512. This arrangement enablestransfer of motor drive torque into the planetary system defined by thespindle, rollers and drum 522. A resistive film potentiometer (notillustrated) can be attached to the casing 506 and moved axially byconnection with the translating drum for position sensing. Optionally, amotor drive control circuit board (not illustrated) can be mountedinside the casing. As described in connection with the first embodimentof actuator 100 above, the actuator 500 is mounted to external structure(not illustrated) in such a way as to be torque-reacting at the fixedend 521 opposite to the output end 520.

In operation, the rollers 524 of the fifth embodiment 500 are positionedin two planes of symmetry—therefore, the motor spindle 512 is always inforce-balance and carries no journal loads, and hence, requires only aminimum thrust load to translate the motor 502 through small distances.This translation is given by d/D X linear distance moved by the drum522. In the case of the 0.5 mm spindle and 10 mm diameter drum exampleabove, this translation will be 1/20th of drum travel. The pressureangle of the roller convolutions is on the order of 20 degrees. Theultimate load carrying capacity of the actuator 500 is governed by theHertzian stress at the eight symmetrical points of contact between thecylindrical portion on the top of the roller convolutions and the drivespindle. From calculations, it appears that, with Hertzian stresses of125 ksi (i.e., approximately half the allowable), and with 4 rollers ina 10 mm diameter shell, axial forces of up to 4 lb. can be produced bythe miniature actuator. This is accompanied by high stiffness, completefreedom from backlash and the potential for high frequency response, ina miniature package less than 0.5 inch in diameter.

Those of skill in the art will appreciate that, in the context ofminiaturized actuators, such as the miniature actuator 500 described inthe above example, it is difficult or practically impossible to achieveshallow helix angles (p with machined helical threads. However, asdemonstrated above, they are readily achievable using directlycontacting disc-rollers 524 skewed at a shallow angle φ, and further,when expressed to the larger diameter D of the drum 522, the desiredhelical structure is readily producible by forming discretehemi-cylindrical corrugations 574 that are capable of reacting axialloads. In additional, the use of the strain energy of a corrugatedthin-walled cylinder, when deflected, advantageously produces adiametral force that, when coupled with the prevailing friction betweenthe rollers and the drive spindle 512, creates the torque required tooperate the device, thereby opening the way to miniaturized planetaryroller devices of high mechanical advantage.

By now, those of skill in this art will appreciate that manymodifications, substitutions and variations can be made in and to thematerials, apparatus, configurations and methods of implementing thelinear actuators of the present invention without departing from itsspirit and scope. Accordingly, the scope of the present invention shouldnot be limited to the particular embodiments illustrated and describedherein, as they are merely exemplary in nature, but rather, should befully commensurate with that of the claims appended hereafter and theirfunctional equivalents.

1. A direct drive electromechanical rotary-to-linear actuator,comprising: an elongated housing; an electric motor, including a statorfixed in the housing and a rotor supported for rotation relative to thestator; and, a planetary drive unit, comprising: an elongated centralshaft coupled to the rotor for conjoint rotation therewith and havingone or more helical threads on an external surface thereof; a planetarynut concentric to the shaft and having a plurality of helical threads onan internal surface thereof; and, a plurality of planetary rollersdisposed concentrically between the central shaft and the planetary nut,each having a helical thread on an external surface thereof that iscomplementary to and in engagement with a thread of the shaft and athread of the nut.
 2. The actuator of claim 1, wherein the central shaftis integral to the rotor of the motor.
 3. The actuator of claim 2,wherein the shaft and rotor comprise a magnetically permeable material.4. The actuator of claim 1, wherein the stator compriseshigh-cobalt-content laminations and the rotor comprises neodymium-ironmagnets.
 5. The actuator of claim 1, wherein the central shaft includesa hollow center, and further comprising a linear variable displacementtransducer (“LVDT”) disposed in the hollow center.
 6. The actuator ofclaim 5, wherein the LVDT comprises: an elongated annular coil sethaving an end fixed relative to a first end of the actuator; and, anelongated rod of a magnetically permeable material slidably disposed inthe annulus of the coil and having a first end fixed relative to asecond end of the actuator.
 7. The actuator of claim 1, furthercomprising: a plurality of axial grooves in the housing adjacent to theplanetary nut; and, a plurality of axial splines on the nut, eachslidably disposed in a respective one of the grooves.
 8. The actuator ofclaim 1, wherein the rotor comprises an elongated annular shaft havingthe planetary nut integrally defined within the annulus thereof.
 9. Theactuator of claim 8, further comprising: an absolute-angular-positionencoder rotatably coupled to the central shaft through a gear.
 10. Adirect drive electromechanical rotary-to-linear actuator, comprising: anelongated housing; a first electric motor, including a first statorfixed in the housing and a first rotor supported for rotation relativeto the first stator; a second electric motor, including a second statorfixed in the housing and an elongated annular second rotor supported forrotation relative to the second stator; a planetary drive unit,comprising: an elongated central shaft coupled to the first rotor forconjoint rotation therewith and having one or more helical threads on anexternal surface thereof; a planetary nut having a plurality of helicalthreads on an internal surface thereof and coupled to the annular secondrotor within the annulus thereof for conjoint rotation therewith and forrelative axial sliding therein; and, a plurality of planetary rollersdisposed concentrically between the central shaft and the planetary nut,each having a helical thread on an external surface thereof that iscomplementary to and in rolling with a thread of the shaft and a threadof the nut; and, means for locking respective ones of the first andsecond rotors against rotation.
 11. The actuator of claim 10, wherein atleast one of the motors comprises a pancake motor.
 12. The actuator ofclaim 10, wherein the rotor locking means comprises a double-armature,double-disc solenoid clutch brake.
 13. The actuator of claim 10, whereinthe rotor locking means comprises: a clutch plate mounted on the firstrotor for conjoint rotation therewith and between the jaws of a caliperbrake; a plurality of rocking levers having rollers disposed atrespective first ends thereof and arranged to move between a firstorientation in which the rollers are engaged in respective slots in anend of the second rotor, and a second orientation in which the rollersare disengaged from the slots; a plurality of first springs biasing therocking levers into the first orientation; a second spring biasing thejaws of the caliper brake together and against the clutch plate; and, aspool moveable between a first position compressing the second springand thereby relieving the bias of the second spring on the caliper brakejaws, and a second position urging the rocking levers against the biasof the first springs and into the second orientation thereof.
 14. Theactuator of claim 13, further comprising: means for releasably holdingthe spool in the first position; and, means for selectably releasing thespool from the first position for movement to the second position. 15.The actuator of claim 14, wherein the means for selectably releasing thespool comprises a solenoid.
 16. The actuator of claim 15, wherein themeans for releasably holding the spool in the first position comprises aplurality of ball bearings disposed in apertures in the spool and heldcaptive in an adjacent circumferential groove by an arm of the solenoid.17. A direct drive electromechanical rotary-to-linear actuator,comprising: an elongated stator housing; an electric motor, including astator supported in the housing for axial movement relative thereto, anda rotor supported in the housing for conjoint axial movement with thestator and rotation relative thereto; and, a planetary drive unit,comprising: an elongated cylindrical spindle having a long axis andcoupled to the rotor for conjoint rotation therewith; a drum disposedconcentric to the spindle and having a thin, cylindrical sidewall with aplurality of helical corrugations therein, the corrugations having apitch of φ relative to the spindle axis; and, a plurality of disc-likeplanetary rollers disposed in a radially symmetrical array about thespindle, each being mounted on the stator housing for rotation about anaxis skewed at an angle φ relative to the spindle axis and having acircumferential surface in frictional engagement with the spindle andconvolutions in the circumferential surface in engagement with arespective one of the corrugations of the sidewall of the drum.
 18. Theactuator of claim 17, wherein the rollers are located in adjacent planesperpendicular to the spindle, each plane containing a radiallysymmetrical array of two or more rollers.
 19. The actuator of claim 17,wherein the circumference of each roller includes two convolutions of apitch given by π D tan φ, where D is the diameter of the drum.
 20. Theactuator of claim 17, wherein the wall of the drum is strained from acylindrical shape to a trochoidal shape incorporating a plurality oflongitudinal zones having a smaller radius alternating with a pluralityof longitudinal zones having a larger radius.